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Measuring light pollution in Beijing and effects on Xinglong Station of National Astronomical Observatory
Baozhou Zhang, Ligen Lu Department of Astronomy, Beijing Normal University
Focus Meeting 21 Mitigating Threats of Light Pollution & Radio Frequency Interference
Contents
Light pollution & night-sky brightness
Measuring night-sky brightness in Beijing Method and instrument
Observations
Data and results
Summary
Light pollution & night-sky brightness
Light pollution refers to any “adverse effect of artificial light including sky glow, glare, light trespass, light clutter, decreased visibility at night, and energy waste.” — International Dark-Sky Association
Urban sky glow
Light trespass
Clutter
Glare
Night-sky brightness Natural sky brightness airglow, zodiacal light, scattered starlight, the Milky Way
Artificial sky brightness poorly designed and excessive outdoor lightings
Measuring night-sky brightness in Beijing
Method Night-sky brightness / luminance
V(mag/arcsec2) = 12.59 – 2.5lgB(cd/m2)*
astronomical unit mag/arcsec2
photometric unit
cd/m2
* Garstang, R. H. 1986, Pub. A. S. P., 98, 364-375.
*candela(cd), one of seven base SI units
Instrument – Night-sky luminance meter
Objective aperture 50 mm Focal length 70 mm
Field of view 2 degrees Sensitivity resolution 10-5 cd/m2
Measurement range 0 ~ 19.99 cd/m2 Measurement error ± 4.6% (± 0.05 mag/arcsec2)
Table 1 Specifications of the Night Sky Luminance Meter. Objective lens
Eyepiece Reflector
Reflector with a hole
PMT A ADC θ
The view seen from the eyepiece
Observation conditions
The necessary conditions to perform all measurements in the survey:
Moonless night Clear night, no clouds or fog The Sun is at least 18 degrees below the horizon (astronomical
twilight) Avoid a site with road lightings or other considerable artificial
lightings nearby
Site 1 – Beijing Normal University (BNU)
Site 1–BNU Site 2
Site 3
Site 4
Site 5
Site 6–Xinglong Station
Site 7–Lingshan
• BNU, Beijing Normal University, 7 km from the city center • Xinglong Station of National Astronomical Observatory, is located about 110 km
northeast to Beijing at an altitude of 950 m. • Lingshan site, at an altitude of nearly 1 km, is about 80 km from the city center.
Night-sky brightness in one night (at Site 1 - BNU) Continuous measurement from earlier night (22:30 p.m. local time) to later
time (04:00 a.m. the next day) Including the zenith night sky and skies at 45 degrees zenith distance in four
directions of north, south, east and west
Figure 2. Variation of night-sky brightness with time.
Observation results
Site 1–BNU
Zenith brightness
Unit 10-3 cd/m2 mag/arcsec2
Max 7.68 17.88 Min 4.65 18.42
Zenith night-sky brightness at various distances from the city center
These brightness data were all measured in late nights, representing the darkest night sky at each observing site.
Observation location
Distance from the city center (km)
Measurement time Weather Night-sky brightness
10-3 cd/m2 mag/arcsec2
Site 1 - BNU 7 6m 16d 01h 20m Clear 4.65 18.42 Site 2 18 8 22 02 30 Clear 2.82 18.96 Site 3 40 7 30 00 00 Clear 1.30 19.81 Site 4 61 8 22 01 30 Clear 0.67 20.52 Site 5 85 8 22 00 00 Clear 0.33 21.29
Site 6 - Xinglong 114 4 3 00 40 Clear 0.26 21.55
Site 7 - Lingshan 80 9 11 02 00 Clear 0.23 21.68
Table 2 Zenith night-sky brightness at seven observation sites.
Beijing
Site 6–Xinglong Station
Site 7–Lingshan
Chengde City
Xinglong Town
Zunhua City
Summary A portable night-sky luminance meter
• Independent of the telescope • With narrow field of view (2 degrees) and high sensitivity
(10-5 cd/m2)
Measuring night-sky brightness in Beijing • Seven observation sites with different distances from the
city center, including Xinglong Station, a major optical site for astronomical observations in China
• Potential risks to night sky of the Xinglong Station: outdoor lightings in nearby cities
Future work Modifications of the night-sky luminance meter
• Remote data transmission and operation
Beijing night-sky brightness monitoring network • Detailed geographical and temporal variations of the night-
sky brightness in Beijing • Assess whether new rules and regulations are necessary to
control light pollution and protect astronomical sites
Thank you for your attention!
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